1. Field of the Invention
The present invention relates to a semiconductor device, in particular, to a package structure of a packaged semiconductor device.
2. Description of the Related Art
In recent years, remarkable spread and expansion of mobile personal computers, tablet personal computers, smartphones, and the like gives an expectation of expansion of a large variety of electronic components to be mounted on those devices. Many of those devices have a feature of combining light weight, small thickness, and compactness by preference to sophisticated and portable designs. Resin mold packages have been thus often used, since the electronic components to be mounted have also been demanded to reduce their size, thickness, and cost. Along with the trend of a shorter product replacement cycle, the package to be mounted has been simultaneously demanded to reduce its size, thickness, and cost. As a result, negative effects have appeared, such as insufficiency in reliability of the product and weakness and low strength of the package. This is because the reduction in size and thickness has been often achieved by shrinking the product without changing or re-examining conventional factors such as a material, a raw material, and a structure. It can be said that changes in encapsulation resin thickness, lead frame thickness, and package substrate thickness, which have been thinned and reduced by the shrinking, are not compensated for. In addition, reduction in cost leads to an effect on difficulty in securing sufficient reliability. Thus, it is necessary to review again the structure and design that can improve the reliability of the electronic components that are reducing in size and thickness. Specifically for the resin mold packages that have been put into production in many semiconductor packages, it is important to realize constant reliability along with the promotion of reduction in thickness and size of the constructing material and the frame, and various approaches are becoming more important for development of semiconductor packages.
FIG. 10 is a sectional view of a package including a resin having a hollow portion and lead frames (FIG. 3 of Japanese Patent Application Laid-open No. 2002-280616). The package includes a resin molded portion 1 and lead frames 2a and 2b, and the lead frames 2a and 2b are retained by the resin molded portion 1. In the hollow portion, one end of the surface of each of the lead frames 2a and 2b is exposed. Another end of each of the lead frames 2a and 2b is exposed outside through the resin molded portion to function as an external terminal. The lead frames 2a and 2b are used in such a manner that an element is mounted onto the lead frame 2b exposed to the hollow portion. The element is fixed onto the lead frame 2b by conductive paste and the like. An electrode provided on the upper surface of the element and the lead frame 2a or 2b exposed to the hollow portion are electrically connected to each other by a wire for use. Examples of the element to be mounted include a photosensor element. In this case, an electromotive force generated by light entering the photosensor element is transmitted from the wire connecting together the upper surface of the photosensor element and the lead frame 2a or 2b to the lead frame to flow to the outside.
However, in the package structure described in Japanese Patent Application Laid-open No. 2002-280616, the lead frames 2a and 2b are retained only by the resin used in the resin molded portion, and hence the retaining force for the lead frames significantly depends on the resin performance. In particular, the structure having the hollow portion has a smaller resin area for retaining the lead frames as compared to the resin encapsulation structure package having no hollow portion. The lead frames 2a and 2b function as external terminals, and hence the terminals are used in a shape obtained through bending and cutting based on a desired embodiment or dimension.
It is known that bending and cutting apply a force most to the root between the resin molded portion and the lead frame, and the lead frame retaining force of the resin is an important factor such as when a force acts in a pulling direction or when a force acts on a twisting direction. During such processing, when the retaining force for the lead frames is weak, the lead frame may dropout from the resin molded portion, or the reduction in retaining force may cause reduction in bending accuracy. Further, the packages are diversifying, and many packages use a lead frame that is thin and fine, as represented by a small-type package, a thin-type package, a multi-pin package, and the like. As the area of the lead frame to be brought into close contact with the resin reduces, the retaining force thereof is further reduced.
Further, the package has a structure having the hollow portion. In the molded portion formed of the resin, a hollow part is a part having the smallest thickness of the resin. Thus, it can be said that the heat resistance and strength against an external force applied to the package are determined based on the resin performance and thickness. As reduction in size and thickness of the package dimension is promoted, the resin thickness of the hollow part is inevitably reduced, and it is difficult to maintain a sufficient strength with the thinned hollow part. As a result, deformation, cracking, and the like are caused due to the insufficiency in strength of the hollow part, which reduces the reliability of the package.
Further, because the hollow part is thinned due to reduction in size and thickness of the package, the above-mentioned lead frame retaining area of the resin is further reduced, which leads to reduction in reliability of the package. As measures against strength reduction and deformation caused by reduction in thickness of the resin, a reinforcement material such as inorganic filler is mixed into the resin. However, mixing filler or the like into the resin causes influences such as, due to reduction in flowability of the resin, reduction in stability during molding, reduction in molding processing speed, and weakness of the molded resin. Thus, the reinforcement of resin by mixing filler is limited.